It is often the case where an electronic circuit comprised of semiconductor (and perhaps other) components must be sealed within a leak-tight chamber so as to protect it from corrosive or otherwise damaging environmental conditions. As an illustrative example, consider a medical implant that requires that electronics must function inside the body for many years. Unless protected, the sodium and other ions that are present within the body would rapidly begin to corrode said electronics, severely compromising the device's longevity. The cardiac pacemaker, for example, is a device which must function under such conditions. As such, the cardiac pacemaker must be sealed within a chamber that is leak-tight (hermetic). While most of the hermetic chamber can be manufactured from a metallic material and thus also serve to complete a cardiac stimulation circuit (the so-called hot-can approach), the electrode placed within or near the heart must be insulated from said hermetic chamber without compromising the hermetic integrity of the pacemaker body containing the electronics. In this example, one electrical conductor must pass through the wall of the hermetic chamber so as to allow for the delivery of therapeutic electrical stimuli via an electrode connected to said electrical conductor on the outside of the hermetic chamber. In most cases, said electrical conductor is encased within a glass, ceramic or other appropriate insulator thereby forming a hermetic feedthrough. Said hermetic feedthrough is then attached to the remainder of the pacemaker body by way of brazing, welding, or other appropriate means. The invention described herein pertains mainly to the fabrication of leak-tight (hermetic) feedthrough assemblies with one or more isolated electrical conductors therein.
In the case of medical implants such as the cardiac pacemaker, the cochlear implant, and other therapeutic devices that must communicate electrical signals originating from, or destined for electrical circuitry within a hermetic chamber, a conductor or multiple conductors are typically oriented within the hermetic feedthrough perpendicular to the wall of said hermetic chamber so as to facilitate the passage of signals. Inherent to this design, in medical or corrosive applications in particular, said conductor or conductors and the insulating material within which they are placed must be comprised of non-reactive, non-corrosive, so-called inert materials such as platinum and ceramic. For example, platinum wire or wires are placed within a ceramic insulator prior to sintering of said ceramic. During the sintering process, the expansion and contraction properties of the two materials are such that leaks along the side of each conductor may occur. In this example, during the sintering process, the platinum expands relatively more than the ceramic thereby leaving behind a hole of greater diameter than the diameter of the conductor intended to fill it. The result is that after the sintering process and subsequent cooling, leaks can be identified. As the quantity of conductors increase, so too does the probability of compromising the hermetic properties of the hermetic feedthrough.
In 1977, de Bruin and colleagues (U.S. Pat. No. 4,050,956) patented a technique for the purpose of joining two or more ceramic bodies. The present invention, while using the natural phenomenon of bonding described within de Bruin's patent, pertains more specifically to the utilization of the aforementioned natural phenomenon of bonding rather than the bonding itself. A principal advantage of the present invention is, by way of the planar and layered structuring, the ability to promote the formation of bonding (mechanical, chemical or otherwise) between the insulator and the metallic conductor thereby improving the hermeticity. This is in contrast to existing methods wherein wires are individually cast into an insulating material in order to form an assembly. As such, no mechanism exists to promote bonding. Indeed in such cases the tendency towards the aforementioned formation of bonding is reduced or eliminated, with the hermeticity achieved only by way of tight dimensions.
Despite the differential expansion and contraction properties of platinum and ceramic, Researchers Allen and Borbidge published work in 1983 that identified that, through the application of compressive force during the sintering process, an improved bond strength can be created between platinum and ceramic (as well as other material combinations). The aforementioned fabrication method with conductors oriented perpendicular to the wall of the hermetic chamber does not lend itself to the application of pressure unless said pressure can be applied in all directions at the same time. While isostatic pressing during sintering is indeed possible, this approach substantially complicates the fabrication procedure thus highlighting one of the advantages of the present invention.